Cable links commonly serve the functions of transmitting electrical power and transmitting electrical signals. When used to transmit signals, the cable links are called data interconnects. When the cable link media is made of an electrically conductive material, it is conventionally called a copper data interconnect, or copper link, whether the metal is strictly copper, or another conductor such as aluminum or an alloy. This convention will be used herein.
Copper cables used in copper links are intrinsically lossy media wherein the signal's higher frequency components are attenuated at higher rates (dB/m) than low frequency components. Attenuation can be reduced, but not eliminated, by using larger gauge wires. Therefore, as the speed of data rate increases to the presently used 3.4 Gbits/s per channel (HDMI), or even 4.8 Gbit/s (for USB 3.0), copper cable wires have become increasingly bulky and expensive, and the overall cable package is unattractive even for transmission distances of less than 5 meters. To compensate for losses in wires, copper transmitter chips have built in pre-emphasis circuitry that amplifies high frequency components of the digital signal before driving the signal over the copper line. On the copper receiver side, a cable equalizer is generally built-in to re-amplify high frequency components (or attenuate low frequency components) of the digital signal. A complete copper link may include the use of either or both pre-emphasis and equalizer.
An example is shown in FIG. 1, in which a transmitter chipset is represented by block 110, a receiver chipset is represented by block 120, and a copper cable 150 connects the transmitter and receiver locations. At the transmitter, input data is received by driver circuitry 112, which operates in conjunction with pre-emphasis circuitry 114 to generate the signal transmitted over cable 150. At the receiver, the received signal is coupled with equalizer 122 and limiting amplifier and driver 124, which produces the output data.
In addition to the limitations and disadvantages already mentioned, the copper link consumes relatively high power, can require expensive EMI shielding, and involves use of substantial amounts of non-recyclable materials.
An optical link can eliminate certain copper link disadvantages, but at a higher initial cost and higher power consumption. The use of fiber cable to directly replace a copper channel requires the addition of an E-O (electrical to optical) transducer and an O-E (optical to electrical) transducer, each of which has to be powered and managed using the existing power sources and control circuitry from the copper transmitter chipset and copper receiver chipset. The E-to-O function for gigabit data transmission via fiber has been traditionally achieved through the use of directly modulated laser diode devices or with external modulation techniques, such as electro absorption modulators or photonic switches. However, these techniques require additional feedback control integrated circuitry (ICs) and additional drivers (optical drivers) that consume substantial power and significantly add to cost. For receiver chipsets where the copper cable equalizer transfer functions are fixed, signal exiting from the OE transducer may need to be re-shaped (i.e attenuation of high frequency component signals) in order to match to the copper cable equalizers.
FIG. 2 shows an example of the FIG. 1 data link in which the copper cable has been replaced by a fiber cable 250 with the typical further required circuitry. The transmitter chipset and receiver chipset correspond to components in FIG. 1 of like reference numerals. In FIG. 2, an electrical-to-optical transducer is represented at 232 and an optical-to-electrical transducer is represented at 262. When the EO transducer comprises a laser diode, or a combination of laser diode and modulator, it requires additional control and driver circuitry, represented at 235. Additionally, because the built-in equalizer of the receiver chipset has a particular transfer function if optimized for losses in copper cable, additional signal re-shaping circuitry (including, for example, a limiting amplifier and/or transimpedance amplifier), represented at 265, may be required to match OE output characteristics to the built-in equalizer. As above-indicated, this additional circuitry is expensive and consumes relatively high power. Therefore, for short distances, (e.g. less than 30 meters), transfer of signal over copper media is generally still preferred due to its relatively lower implementation cost.
It is among the objects of a first aspect of the invention to provide a solution to the problems and limitations associated with converting a copper data link to a data link using an optical cable.
The bulk of high speed transmission based on copper data links or interconnects utilizes differential signaling methods. In differential signaling, two signals, identical in magnitude but exactly 180° out-of-phase, are used in order to maintain signal integrity. Since all data processing and data generation has its roots in integrated circuits, which are electrical devices and therefore generate electrical signals, copper based transmission utilizing differential signaling is the dominant method of data transfer for electrical systems. Existing differential signaling is illustrated in the simplified diagram of FIG. 3. At copper driver 330, the differential signal comprises data signal V+ and data signal V−, and these are coupled over copper transmission line or link 350 which, in this example, also carries ground reference potential.
When trying to establish or extend a high speed data interconnect over relatively long distance, an optical fiber based interconnect utilizing a diode emitter such as a laser, VCSEL, or light emitting diode, may be used to extend the transmission line of the copper interconnect. As indicated previously, the optical high speed data interconnect starts with a copper driver and eventually ends with a copper receiver, since all present data systems originate and terminate from and into electrical processes.
In FIG. 4, a schematic of an existing optical interconnect link extender is illustrated. Shown in the diagram are copper driver 430, equalizer 435, optical driver 440, diode emitter 445, fiber optical waveguide 450, diode detector 455 transimpedance amplifier 460, limiting amplifier 470, and copper receiver 480. The Figure demonstrates that since diode light emitters are single ended devices, only one of the data signals (V1+) is used while the other signal is terminated (wasted), via 50 ohm resistor 439 in this example. The signal V1+ from copper driver 430 is passed though equalizer 435, generating a conditioned output V2+ which is fed into optical driver 440. The optical driver converts the input voltage signal into an equivalent current signal (I1+). This step is necessary since present state of the art VCSELs, lasers and LEDs are operated as current driven devices. The current signal is fed into the diode emitter, generating photon signals. The emitted photon signals can then be coupled into optical waveguide 450 (i.e. fiber) or simply via free space. At the output end of the optical waveguide is diode detector 455 which converts photons into photocurrent (I2+). The photocurrent is coupled to transimpedance amplifier (TIA) 460 which converts the photocurrent into an amplified voltage signal and also converts the single ended signal into a differential signal (V3+ and V3−). The differential signal is fed into limiting amplifier 470 to further amplify the signal (V4+ and V4−). Finally, the amplified differential signal is coupled into a copper receiver, completing the data transmission.
It is among the objects of a further aspect of the invention to provide improvements to high speed electro-optical data interconnects of the type just described, including making them more efficient and less expensive.